It looks like there's a bit of a hiccup with the Mario + Rabbids Kingdom Battle Ultra Challenge Pack for some people. If you purchased the Season Pass, you should automatically get the content, but for some people, the content isn't downloading. Nintendo is aware of the issue, and while they work on a fix, they say you might have to go to the eShop and re-download the Season Pass to get things to work. Thanks to Alias79 for the heads up!